1. Field of the Invention
The present invention relates to a gas-leak detector which is suitable for use as, for example, a gas-leak detector for detecting a leakage of carrier gas of a gas chromatograph (hereinafter abbreviated as GC); which includes a suction pump and gas detection sensors that are small and integrated together to reduce the size and weight of the gas-leak detector and facilitate operation of the gas-leak detector; which has a cell for receiving sucked gasses that has a small capacity so that the gas detection sensors accurately and quickly respond to the gasses and the gas-leak detection sensitivity can be increased; in which pulsation of the suction pump is reduced so that stable detection operation can be achieved; and which can be manufactured at a low cost.
2. Description of the Related Art
In general, a GC includes a sample introduction unit and a separation column that are connected to a pipeline of carrier gas, such as helium gas. The sample introduction unit introduces a sample into a flow of the carrier gas, which carries the sample to the separation column. Components of the sample are separated by the separation column, and the separated components are detected by a detector.
A leakage of the carrier gas from a flow channel that extends from the supply pipeline of the carrier gas to the detector adversely affects the analytical accuracy. If air enters the flow channel at the location of the leakage, the separation column deteriorates at an accelerated pace. Therefore, it is necessary to regularly perform carrier gas leak tests. Gas-leak detectors are used for these tests.
Japanese Unexamined Patent Application Publication No. 9-292302 and Japanese Utility Model Registration No. 3127573, for example, describe examples of gas-leak detectors which include inlets for a probe gas, which is a sample gas, and a reference gas; pipelines that communicate with the inlets; a pump that sucks the gasses; and thermal-conductivity-type gas sensors that detect the gasses. Each gas sensor includes platinum coils connected to a bridge circuit. In gas leak detection, the reference gas, such as air, and the probe gas are sucked and the sensors respond to the sucked gasses. The bridge circuit electrically detects a change in the electric resistances of the sensors, the change corresponding to a change in the temperatures of the sensors caused by a difference between the thermal conductivities of the gasses.
However, since the above-described gas-leak detectors include platinum coils as sensor elements, cells that house the coils have a large capacity, and a large and heavy cell block is used. In addition, since the pump is large and has a large suction capacity, there is a limit to the extent to which the detection sensitivity can be increased, and it is difficult to stabilize the detection operation because of large pulsation and noise. Furthermore, the gas-leak detectors are large and heavy since the pump and the cells that house the sensors are separately arranged.
These problems may be solved by reducing the size of each sensor. Japanese Unexamined Patent Application Publication No. 2000-258376, for example, proposes a chip sensor including a microheater manufactured by silicon micromachining technology so that the microheater is capable of heating a small area of about several tens of micrometers square. The chip sensor may be used as a gas sensor.
Another way to solve the above-described problems is to reduce the size and suction capacity of the pump and stabilize the operation of the pump. This may be achieved by a gas-leak detector that sucks the sample gas and the reference gas through a pumping operation of a diaphragm that is vibrated by a direct-current motor.
However, the pump is still large and has a large suction capacity. Therefore, there is still a limit to the extent to which the detection sensitivity can be increased and it is difficult to stabilize the detection operation because of large pulsation and noise. In addition, the gas-leak detector is large and heavy since the pump and the cells are separately arranged and require individual installation spaces. Furthermore, tailing occurs at the beginning and end of the detection operation, and the detection time and the waiting time required are long when the detection is repeated.
Japanese Unexamined Patent Application Publication Nos. 2009-97393 and 2005-307858, for example, respectively describe a small piezoelectric microblower and a small piezoelectric diaphragm pump that solve the above-described problems of the pump. According to these publications, a piezoelectric element is disposed on one side of a diaphragm, and is connected to a drive device, such as an oscillation circuit. The drive device is operated so as to apply a pulse voltage having a constant frequency to the piezoelectric element and cause the diaphragm to resonate. A blower chamber capable of performing a pumping operation is provided between the diaphragm and a partition plate. The blower chamber communicates with two flow channels that allow fluid to be supplied to and discharged from the blower chamber.
Although the sizes and weights of the sensors and pump, the amounts of sucked gasses, and the capacity of the cells can be reduced by using the above-described microheater and the piezoelectric microblower, it is difficult to achieve a desired result unless the reductions are made in coordination with each other.
For example, even when the size of the sensors and the suction capacity of the pump are reduced, if the capacity of the cells which receive the gasses is too large relative to the size of the sensors and the suction capacity of the pump, the gasses diffuse in the cells and the concentrations thereof decrease. As a result, the desired detection sensitivity cannot be obtained and the response time of the detection operation increases.
Japanese Unexamined Utility Model Registration Application Publication No. 3-60060, for example, describes a detector that is arranged downstream of a column of a GC to detect the components of a sample gas on the basis of thermal conductivities thereof after the components are separated by the column. The detector includes a thin sheet body having a pair of left and right irregular-shaped punch holes. Plate-shaped detector bodies are arranged on either side of the sheet body, and the detector bodies and the sheet body are fastened together with screws. One of the detector bodies has gas inlet holes and gas outlet holes at either side thereof, the gas inlet and outlet holes communicating with the punch holes. Detector attachment holes are formed at intermediate positions between the gas inlet holes and the respective gas outlet holes, and small chip sensors are housed in the attachment holes. The sample gas discharged from the separation column is introduced into one of the gas inlet holes, and carrier gas, which serves as reference gas, is introduced into the other one of the gas inlet holes. The gasses are guided to the respective chip sensors, and the thermal conductivities of the gas components are measured and subjected to comparison to detect the gas components and determine the concentrations thereof. Subsequently, the gasses are discharged through the gas discharge holes.
The above-described detector includes the sheet body in which the pair of irregular-shaped punch holes, which serve as gas passages, are formed at separate positions. The punch holes are provided with the respective gas outlet holes. The detector attachment holes, which have a large diameter, are formed at the intermediate positions of the punch holes. Therefore, it is difficult to form small and uniform punch holes, and the sheet body and the detector bodies have large and complex structures. In addition, since the punch holes have large capacities and dead volumes, diffusion of the introduced gas components occurs. Therefore, the sensing accuracies of the chip sensors are reduced and the desired sensitivity and detection accuracy cannot be obtained.